Acid rain is attributable to the release of nitrogen oxides and sulfur oxides into the atmosphere. The nitrogen oxides (NOX) and sulfur oxides (SOX) are primarily derived from combustion processes such as petroleum in internal combustion engines of automobiles and from utilities which burn coal or high sulfur fuel oil in the furnaces of the steam-electric generating stations. Means have been devised and implemented for the abatement of NOX and SOX from automobile exhausts but the coalburning electric generating stations remain a severe domestic and international problem. Unlike petroleum, coal cannot be effectively desulfurized or denitrogenated. As a consequence, it is necessary to remove the NOX and SOX from the flue gas. There are scrubbing processes employing pulverized limestone or NaOH solution. In both cases there are severe disposal problems; magnesium and calcium sulfate when lime is used and sodium sulfate when sodium hydroxide is used. Magnesium sulfate is Epsom Salt, a purgative, calcium sulfate forms a gel-like material presenting a severe problem of disposal. Sodium sulfate like any other soluble material would enter the aquifer and the river system as an objectionable contaminent.
In the U.S. there is burned in the generation of electrical energy over 660 million tons of coal per year with an average sulfur content estimated to be 2%. The burning of the coal produces sulfur dioxide and sulfur trioxide equivalent to approximately 30 million tons of sulfuric acid and nitrogen oxides equivalent to approximately 6 million tons of nitric acid per year.
These acid gases enter the atmosphere and are in the course of being transported by the wind, exposure to sunshine, and moisture, converted to sulfuric and nitric acids and fall as acid rain. It is a well recognized but unsolved problem. It has been addressed by congress and has been the object of research effort sponsored by the Department of Energy, Electric Power Research Institutes, and private utilities. As well as it being a national problem, it is also an international problem resulting from the effects that the acid rain has on architecture, statuary, lakes, and vegetation. To date no satisfactory solution has been derived.
There are solutions, but they have many problems either because of high cost or incomplete or inadequate removal of the sulfur or nitrogen oxides. These processes fall into four different categories identified as follows:
First is simple washing of the coal. This is practiced on approximately 40% of the coal and removes a small percent of the sulfur but none of the nitrogen oxides. Furthermore, the washing operation introduces pollution into the waterways into which the wash water is discharged. It is probably the poorest and most incomplete of any of the possible solutions. The next process grouping is one in which the coal is in various ways mixed with pulverized limestone and the limestone during combustion on the furnace grate removes the sulfur as calcium or magnesium sulfates. A related process is to use an alkali carbonate instead of limestone. These processes are expensive, ineffective in removing the nitrogen oxides, only partially effective in removing the SO.sub.2, and present extreme problems of disposal of the sulfate product. Additionally, the abrasiveness of the coal ash and the limestone is detrimental to the furnace components. Furthermore, it increases the load on the dust removal system, which are the cyclone separators and the electrostatic precipitators. The fourth process involves a wet slurry scrubber system in which a finely pulverized limestone, magnesium oxide or sodium hydroxide is used to scrub the boiler flue gas. This process in general has many of the same weaknesses of those already described, namely, failure to remove the nitrogen oxides and only partially effective in removing the SOX. The system is extremely corrosive and abrasive and as a consequence maintenance is extremely high. A related process has recently been announced as a result of laboratory scale tests. This process is very similar to the limestone slurry scrubbing, but adds an emulsion of yellow phosphorous to the scrubbing slurry. No cost figures were reported, but a rough approximation would classify this to be hazardous because of the elemental yellow phosphorous which is spontaneously combustible when dry and is extremely corrosive to the skin. These hazards are in addition to the other shortcomings of process such as abrasiveness and corrosiveness making for prohibitive maintenance costs.
It can be seen that none of the above remove in a single step the nitrogen oxides, and, as a consequence, the nitrogen oxide must be removed in separate operations such as the addition of ammonia and passing the nitrogen oxide-ammonium mixture over a precious metals catalyst or a titania vanadium oxide catalyst which reduces a portion of the nitrogen oxides. Here again, however, the removal of the nitrogen oxides is only about 70% or less complete.